Particle size pigmented media

With all pigments, the particle size distribution of the pigment as synthesised is the most important single factor in ensuring its optimum potential performance in use, but this potential can only be achieved if the particles are properly dispersed. To understand why particle size is so important it is necessary to look in some detail at the optical behaviour of tiny pigment particles dispersed in a transparent organic medium. The exact nature of this medium depends on whether the end-use is a paint, a printing ink or a plastic. 

The ultimate viscosity T) of a pigmented medium is practically independent of the particle size, but increases appreciably with rising pigment volume concentration and higher vehicle viscosity [114],... 

In order to manufacture a diarylide yellow pigment, the diamine, dissolved in hydrochloric acid or sulfuric acid, is bisdiazotized with an aqueous sodium nitrite solution. The resulting diazonium compound is subsequently coupled onto two equivalents of acetoacetarylide. Since a material with a very fine particle size is needed for a fast and complete coupling reaction, the coupling component is prepared by dissolution in an alkaline medium and reprecipitation with an acid, such as acetic acid or hydrochloric acid. Dichlorobenzidine cannot be diazotized stepwise. A lack of sodium nitrite does not produce the monodiazonium compound, but an excess of 3,3 -dichlorodiaminodiphenyl remains. 

P.R.252 provides yellowish to medium red shades and is recommended particularly for use in architectural paints. The pigment shows very poor fastness to a number of organic solvents which are commonly used in paints, a deficiency which largely precludes it from being used in oven drying systems. Regarding lightfastness and weatherfastness, the only available type with coarse particle sizes performs somewhat better than the much more yellowish P.O.5. 

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